Despite launching his campaign more than a year ago, the 42-year-old Mr. Newsom had been trailing in the polls and in fund raising behind Jerry Brown, the state attorney general. Mr. Brown hasn’t yet declared his candidacy for governor, though he launched an exploratory campaign for the position last month.

Mr. Brown had $7.4 million in campaign cash after the January to June fund-raising period, while Mr. Newsom had only $1.2 million. A Field Poll released earlier this month gave Mr. Brown a 47% to 27% advantage over the San Francisco mayor for the June 2010 Democratic primary.

“With a young family and responsibilities at city hall, I have found it impossible to commit the time required to complete this effort the way it needs to — and should be — done,” Newsom said in a statement.

His withdrawal leaves Jerry Brown, the state’s attorney general and a former governor, as the leading Democratic contender to succeed Republican Arnold Schwarzenegger, who is prohibited from running again by California’s term-limits law.